An electric machine such as an electric motor, power generation system, genset, or the like, is generally used to convert one form of energy into another and may operate in a motoring mode to convert electrical input into rotational or otherwise mechanical output, or operate in a generating mode to convert rotational or otherwise mechanical input into electrical output. Among the various types of electric machines available for use with an electric drive, switched reluctance (SR) machines have received great interest for being robust and cost-effective. A typical SR machine includes a rotor and a stator, each of which may include a plurality of poles. During operation such as in the motoring mode, a rotational field is applied to the stator, which, through the magnetic reluctance effect, “pulls” the rotor along, thus generating the rotor torque.
Among other factors, proper determination of the position and speed of the rotor of the SR machine during relatively low speed operations may have significant impacts on overall performance and efficiency. Some conventional control schemes rely on mechanically aligned speed wheels and sensors to detect and determine the position of the rotor relative to the stator at machine standstill or low speed operations. However, such sensor-based control schemes typically require costly implementations and are susceptible to error. For instance, an error of 2 degrees in the detected mechanical rotor position of an SR machine, caused by a skewed sensor, a mechanical misalignment of the speed wheel, or the like, may correspond to a 0.5% decrease in efficiency of the electric drive assembly at full load.
Sensorless control schemes can also be used to derive the rotor position using electrical characteristics of the SR machine. For example, the control system of U.S. Pat. No. 5,525,886 to Lyons, et al. injects a current signal to compute a total voltage flux in the SR machine. Lyons then determines the rotor position based on the voltage flux and the phase current. While Lyons may provide more simplicity over sensor-based schemes, Lyons' method is susceptible to noises resulting from a pulsating torque created by the injected pulses, especially in a light-load condition when the pulsating torque causes some noticeable torque ripples that would impact the performance. The performance impact may lead to larger position and speed estimation deviation or error that would cause the electric machine to run in an opposite direction or be locked at a lower or higher speed than the target speed.
The present disclosure is directed to overcoming or mitigating one or more of these problems set forth.